im not sure but id say that it is either a bad temperiture sensor or it is actually that hot.Either way thats probably whats turning the system off because the mobo has an on board safety feature telling it to turn off when the comp gets to a high temperature.
Have you checked that all your fans are spinning if there is one that isnt then that may be the cause of the high temps/random restarts
another thing to try temporerally would be to run the system with the side panel of the case off